(1) Field of the Invention
The present invention relates to a delay circuit in, for example, an electrically erasable and programmable read-only memory (E.sup.2 PROM), a nonvolatile random access memory (NOVRAM), and the like.
(2) Description of the Related Art
In an E.sup.2 PROM, NOVRAM, and the like, the write/erase voltage V.sub.PP is remarkably higher than the normal power supply voltage V.sub.CC (for example, 5 V). Such a write/erase voltage V.sub.PP is 20 V to 25 V. In the prior art, this write/erase voltage V.sub.PP is supplied from the exterior. However, in recent years, a step-up circuit has been provided in each chip to generate an internal write/erase voltage, thus allowing the external power supplies and external terminals (pads) of the chips to be reduced.
During a write/erase mode, a clock signal is supplied to the step-up circuit, thereby increasing the write/erase voltage V.sub.PP. As a result, this high voltage V.sub.PP is applied to a memory cell to perform a write-erase operation thereupon due to a tunneling effect.
However, such a high voltage V.sub.PP rises too rapidly, and the cell to which the write/erase voltage V.sub.PP is applied may be destroyed or the life-time thereof shortened due to the application of an excessively large electric field to the tunneling films. To avoid this, a delay circuit is incorporated in a chip for slowing the rise of the write/erase voltage V.sub.PP. That is, such a delay circuit blunts the waveform of the write/erase voltage V.sub.PP.
A prior art delay circuit comprises an N-channel depletion-type transistor, linked between a signal input terminal and a signal output terminal, a N-channel depletion-type transistor as a load, and a capacitor C connected to the input terminal, thereby forming a capacitance-resistance (CR) time constant circuit.
However, in the above-mentioned prior art, it is difficult to precisely increase the CR time constant. If the CR time constant is caused to be increased, the manufacturing cost of the circuit is increased, which will be later explained in more detail.